1. Field of the Invention
The present invention relates to an analog-to-digital (hereinafter also "A/D") conversion device that converts an analog signal fed thereto into a digital signal and outputs the digital signal, and more particularly to an A/D conversion device having an input level adjustment function for adjusting the DC (direct-current) level of the analog signal fed thereto.
2. Description of the Prior Art
In analog-to-digital conversion, a fluctuation in the DC level of an input analog signal causes a distortion in the digital signal obtained as a result of analog-to-digital conversion. This is because analog-to-digital conversion can cope with a limited range of voltages.
To prevent such distortion, some A/D conversion devices are provided with an input level adjustment function for adjusting the DC level of the analog signal that is to be subjected to analog-to-digital conversion. The circuit configuration of a conventional A/D conversion device having such an input level adjustment function is shown in FIG. 3. As shown in FIG. 3, this A/D conversion device is composed of an operational amplifier 1, resistors 2 and 3, an A/D converter that performs A/D conversion on a signal fed thereto and outputs the resulting signal, and a variable constant voltage source 50 that produces a constant voltage that can be varied.
To the non-inverting input terminal (+) of the operational amplifier 1, an analog signal fed in via a signal input terminal T.sub.1 is fed. On the other hand, the inverting input terminal (-) of the operational amplifier 1 is connected through the resistor 2 to the output terminal of the operational amplifier 1 itself, and is connected also through the resistor 3 to the variable constant voltage source 50. The signal output from the operational amplifier 1 is subjected to A/D conversion by the A/D converter 4, and is then output via terminals T.sub.O1, T.sub.O2, . . . , T.sub.On.
With respect to the operational amplifier 1, negative feedback is achieved through the resistor 2. Accordingly, if it is assumed that the voltage fed in via the terminal T.sub.I is V.sub.I, the resistances of the resistors 2 and 3 are R.sub.f and R respectively, and the constant voltage output from the variable constant voltage source 50 is V.sub.B, then the voltage V.sub.A of the analog signal fed to the A/D converter 4 is given by EQU V.sub.A ={(R.sub.f +R)/R}V.sub.I -(R.sub.f /R)V.sub.B
Accordingly, by adjusting the constant voltage V.sub.B of the variable constant voltage source 50, it is possible to adjust the DC level of the analog signal fed to the A/D converter 4. In this way, even if a fluctuation occurs in the DC level of the analog signal that is fed in via the terminal T.sub.I so as to be subjected to A/D conversion, or even if an offset occurs in the output of the operational amplifier 1, it is possible to prevent the output digital signal from being distorted.
However, in this conventional A/D conversion device, to vary the DC level of the voltage V.sub.A of the analog signal fed to the A/D converter 4 by .DELTA.V.sub.A, the constant voltage V.sub.B of the variable constant voltage source 50 needs to be varied by .DELTA.V.sub.B, which is given by EQU .DELTA.V.sub.B =-(R/R.sub.f).times..alpha.V.sub.A
Thus, depending on the values at which the resistances R.sub.f and R are actually set, the voltage V.sub.B of the variable constant voltage source 50 needs to be adjusted within an extremely narrow range (more specifically, the farther R.sub.f is higher than R, the narrower the range of adjustment), which makes the adjustment of the voltage V.sub.B of the variable constant voltage source 50 extremely difficult.